1. Field of the Invention
This invention relates to refractory and other compositions, and more particularly to castable refractory compositions comprising particulate refractory aggregates and binder components. Compositions for iso-pressing are also within the ambit of the present invention.
2. Description of Related Art
Castable refractory compositions comprising particulate refractory aggregates and binder components are used in the metallurgical, cement, petrochemical and glass making industries for the production of furnace and ladle linings, launders or runners, pouring nozzles and other heat resisting applications.
Castable refractory compositions are generally produced by their manufacturers at sites remote from the premises of the end-users. In a conventional process for producing a castable refractory composition, raw materials, either in bulk, intermediate bulk containers, or bags on pallets comprising particulate refractory aggregates and binder components, are charged into a mixer, which is normally of around one to two tonnes capacity. With the exception of the pallets, which may be re-used, raw materials packaging is frequently subsequently discarded, especially when small paper or plastic bags are involved. Raw materials packaging is normally disposed of by land-fill or incineration, both of which are expensive processes, the former being subject to state imposed taxes. However, residues of particulate refractory materials in the packaging also lead to waste, and therefore a reduction in this and the volume of redundant packaging which is required to be disposed of would be highly desirable.
The batch of discharged raw materials is normally subjected to a thorough mixing operation in the mixer for 5-20 minutes, depending on the composition, in order to produce a homogeneous composition. The power consumption of the mixer is considerable and adds to the costs of the product. Finally, the composition is discharged from the mixer and re-packaged in 25 Kg, 250 Kg, 500 Kg, 1000 Kg or 2000 Kg bags as required, and after having undergone extensive quality control testing, is transported to the premises of the end-user.
On arrival at the end-user's premises, the various types of bags are broken open, and the contents discharged into a second mixer. The bags may again be discarded, with the attendant disposal problems set out above, and the composition is subjected to further mixing in the second mixer, during which process, water is added to the mixed composition. The castable composition is then discharged from the mixer for conveying, or pumped, to the site where the cast lining is formed in-situ.
It will be apparent from the above description that there are several difficulties inherent in the current method for the manufacture of castable refractory compositions. In addition to the disposal problems represented by the two sets of discarded bags, the entire composition is required to be mixed on two separate occasions, in large mixers having a substantial power consumption.
Having mixed aggregates with binder, it is important that the composition remains dry until the time that it is required. Otherwise the binder can deteriorate preventing homogenous curing of the final mixtures. It is for this reason, primarily, that smaller bags of the composition might frequently he employed, so that if any bag does develop a leak during storage or transportation, permitting ingress of water, only a small amount of the composition is ruined.
However, a completely unrelated issue is that furnaces cost a considerable sum of money to he kept idle while repairs are effected. Furnace operators plan upgrades and repairs to the furnace and related equipment meticulously, so that the time for which the furnace is inoperative may be minimised. One repair is the replacement of the lining of troughs, runners, furnaces and ladle linings etc, and for the refractory supplier, who usually has the responsibility of overseeing the repair, this means little time can be allowed for the item which is to be repaired to cool before the repair must be effected so that the newly cast refractory composition will be cured and hardened before the furnace is switched back on line. Time is therefore of the essence. Furthermore, when casting refractory compositions into still very hot components which have only recently come off line, the cure rate of the composition is inevitably very fast. While this in itself is not a problem, it means that when boundaries exist between one cast and a subsequent cast, lines of weakness at the boundary can develop resulting possibly in cracks or other defects in the refractory lining.
The answer to these problems is to cast in large volumes, very quickly. Thus furnace operators may be provided with a large two-tonne mixer and a number of mobile skips or hoppers of the same capacity so that they may have the mixed composition discharged into the skips one immediately following the other. Thus, perhaps three or four skips might be filled one after the other, so that each may be discharged in rapid succession giving, to all intents and purposes, a continuous delivery. Meanwhile, the mixer is mixing a further batch, which may be ready for emptying into the first (now empty) skip or hopper, before the third skip has been discharged. In this way, perhaps up to sixteen tonnes of refractory may be cast almost at one time.
However, charging, adding water, mixing and discharging mixers is relatively time consuming, and it is this element in the process which is the bottleneck. Eventually the speed of the cycle is reduced to its slowest component and the quasi-continious casting reduced to batch casting. Since it. is the object is to have a continuous cycle of charge, mix and discharge, so that the casting process can be continuous, the mixing process needs to be speeded-up.
This militates against the use of small bags of composition simply because of the time required to empty the bags into the mixer. Thus it is an object of the invention to overcome the aforementioned problems, or at least to mitigate their effects.